Online, I use onelook.com and Cambridge mostly. On CD, I go for Oxford, but I am using monolingual versions- the Oxford Shorter is excellent, and the full one is fantastic, but rather expensive.
Have a look at these two- they come from Oxford and Merriam-Webster, so they should be of a high standard.
http://www.usingenglish.com/amazon/us/0877794723.html http://www.usingenglish.com/amazon/us/0198608780.html